In general, the present invention relates to clays and more particularly to a method for improving the brightness of clays. The present invention is primarily concerned with a method for improving the brightness of kaolin clays in which discolored titanium and iron impurities are removed by gravity settling or centrifugation from a clay slurry.
Natural occurring clays vary considerably in their color properties, even when produced from mines in the same locality or even from different sites in the same mine. Naturally occurring kaolin clay deposits contain discoloring contaminants, for example iron and titanium minerals. Titanium and iron minerals in kaolin usually occur as discolored particles and these are largely responsible for the yellow-brown shade of many kaolins. Often a clay is rejected as being unsuitable for commercial use solely on the basis of its color even though its other physical properties such as the viscosity of its clay-water slurries and its particle size distribution are within desired limits.
The prior art in this area has suffered from its inability to easily and economically correct the color properties of a given clay to those desired. A great deal of difficulty has been experienced in achieving the desired brightness. This difficulty arises in part from the fact that the titanium and iron contaminants in kaolin clay are difficult to alter in color or to remove.
Clays from mines are generally crushed and then slurried to produce a clay slip. The clay slurries are often treated with a peptizing agent in order to achieve minimum viscosity, or a maximum dispersion of clay particles in a slurry to facilitate fractionation to the desired particle distribution. Among the well known peptizing agents utilized in the clay arts are polyphosphates, carbonates, silicates, alkalies in general, mixtures thereof, basic salts, and the like. The amounts of these peptizing agents used in the original deflocculation of the clay slurries may vary from 0.5 pounds per ton of dry clay to 8 pounds per ton of dry clay, the exact amount depending on the clay system itself and the type of peptizing agent used. These slips or slurries are then classified to the desired clay fraction by such methods as hydroseparation, centrifugal forces and the like.
Under these circumstances the brightness of kaolin clays may usually be increased by fractionation, that is, by the preparation of fine fractions of higher surface area. The finer the particle size the brighter the clays. However, this increase in brightness is usually insufficient. The conventional methods of classification of clay slurries normally increase the clay brightness by about 0.5 to 3.5 points above that of the original clay. It should be noted that the mere step of preparing fine fractions does not necessarily reduce the content of discoloring contaminants. In order to be acceptable commercially, additional treatment is often required of the refined clays, such as chemical leaching. Leaching of the clay with chemicals such as zinc or sodium hydrosulphite generally results in improved brightness of the refined clay slurries, but the increase in brightness is ordinarily only on the order of 2 to 5 brightness points.
These clays are then generally filtered and dried, which places the beneficiated clays in a condition for commercial use. Thus, with the normal beneficiating procedures of the clay industry, one is able to prepare clays for commercial use having a brightness not more than around 88 points.
There are other methods of improving the brightness of clays but generally they are excessively expensive and do not give a sufficient increase in brightness to justify the expense.
The use of dispersants as described above produces a state known as deflocculation. It is well known that an excess of peptizing agents can result in an undersirable increase in viscosity because of the formation of a reflocculated state and that if the peptizing treatment is carried too far an intractable gel state will be produced.
Thus, prior to the discovery of Maynard, Skipper and Millman described in U.S. Pat. No. 3,371,988, the prior art practices scrupulously avoided the reflocculated state and attempted to maintain a condition of minimum viscosity during classification. Maynard, et al, discovered that the reflocculated state had unique utility because it creates a system which results in the preferential separation of titanium contaminants, resulting in a significant increase in purity and brightness of the refined kaolin fraction. As described in U.S. Pat. No. 3,371,988, Maynard, et al, discovered that the preferential separation of titanium contaminants could be achieved by adding an excess of peptizing agents beyond the amount required to produce minimum viscosity for classification to deliberately create a reflocculated slurry in which liberation and sedimentation of titanium impurities from the clay particles would spontaneously occur.
The above patented discovery led to additional attempts to discover additional materials which might produce improved effects with further increase in brightness.
Accordingly, it is an object of the present invention to provide a new, unobvious and highly effective method of beneficiating clay which overcomes the deficiencies of the prior art as described above.
It is a further object of the present invention to provide a method of producing clays with a higher brightness than possible by conventional prior art treatments of the same clay.
Other objects and a fuller understanding of the present invention may be had by referring to the following description and claims taken in conjunction with one another.